Assuming your GUI is Gnome Shell, please...

1. Log in via SSH while the GUI is still frozen.

2. Run:  killall -ABRT gnome-shell

3. Follow these instructions:
   https://wiki.ubuntu.com/Bugs/Responses#Missing_a_crash_report_or_having_a_.crash_attachment

However if that all doesn't work then don't panic. It appears the problem might be a kernel crash that happened in the Intel graphics driver:

97113.540137] general protection fault: 0000 [#1] SMP PTI
[97113.540140] Modules linked in: nf_conntrack_ipv6 nf_defrag_ipv6 xt_tcpudp ipt_MASQUERADE nf_nat_masquerade_ipv4 nf_conntrack_netlink nfnetlink xfrm_user xfrm_algo iptable_nat nf_conntrack_ipv4 nf_defrag_ipv4 nf_nat_ipv4 xt_addrtype xt_conntrack nf_nat nf_conntrack libcrc32c br_netfilter bridge stp llc overlay aufs ip6table_filter ip6_tables iptable_filter binfmt_misc snd_hda_codec_hdmi snd_hda_codec_realtek snd_hda_codec_generic eeepc_wmi asus_wmi snd_hda_intel sparse_keymap wmi_bmof mxm_wmi snd_hda_codec snd_hda_core snd_hwdep intel_rapl snd_pcm x86_pkg_temp_thermal intel_powerclamp kvm_intel kvm irqbypass crct10dif_pclmul crc32_pclmul ghash_clmulni_intel pcbc snd_seq_midi snd_seq_midi_event snd_rawmidi snd_seq aesni_intel snd_seq_device snd_timer aes_x86_64 crypto_simd glue_helper cryptd snd mei_me
[97113.540168]  intel_cstate intel_rapl_perf joydev input_leds soundcore shpchp mei wmi mac_hid acpi_pad sch_fq_codel coretemp parport_pc ppdev lp parport ip_tables x_tables autofs4 hid_generic usbhid hid uas usb_storage i915 i2c_algo_bit drm_kms_helper syscopyarea e1000e sysfillrect sysimgblt fb_sys_fops ptp i2c_i801 drm nvme pps_core r8169 ahci nvme_core mii libahci video
[97113.540187] CPU: 3 PID: 15181 Comm: code Not tainted 4.15.0-45-generic #48-Ubuntu
[97113.540188] Hardware name: System manufacturer System Product Name/STRIX H270F GAMING, BIOS 0811 09/27/2017
[97113.540210] RIP: 0010:gen8_ppgtt_set_pde.isra.43+0x45/0x60 [i915]
[97113.540212] RSP: 0018:ffffa40fcce1b8d0 EFLAGS: 00010206
[97113.540214] RAX: cd9e819870096ec8 RBX: 0000ffffbb200000 RCX: ffff8eeb5bf7c5c0
[97113.540215] RDX: 00000003a2c64003 RSI: ffff8ee8f3da7fa0 RDI: ffff8eeaa253e000
[97113.540216] RBP: ffffa40fcce1b8d0 R08: 0000000000000000 R09: 0000000000000000
[97113.540217] R10: 0000000000000000 R11: 0000000000000000 R12: 00000000000001d9
[97113.540218] R13: 0000000000040000 R14: ffff8eeaa253e000 R15: ffff8eeb843a8000
[97113.540220] FS:  00007f5b1947c540(0000) GS:ffff8eec76cc0000(0000) knlGS:0000000000000000
[97113.540221] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[97113.540222] CR2: 00007f5b00de6000 CR3: 00000003441f4003 CR4: 00000000003606e0
[97113.540224] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[97113.540225] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[97113.540226] Call Trace:
[97113.540241]  gen8_ppgtt_alloc_pdp+0x1c2/0x340 [i915]
[97113.540255]  gen8_ppgtt_alloc_4lvl+0x56/0x150 [i915]
[97113.540267]  ppgtt_bind_vma+0x39/0x70 [i915]
[97113.540282]  i915_vma_bind+0x75/0xd0 [i915]
[97113.540296]  __i915_vma_do_pin+0x2d7/0x490 [i915]
[97113.540309]  eb_lookup_vmas+0x7b3/0xb40 [i915]
[97113.540312]  ? __pm_runtime_resume+0x5b/0x80
[97113.540324]  i915_gem_do_execbuffer+0x49c/0x1120 [i915]
[97113.540327]  ? ttwu_do_wakeup+0x1e/0x140
[97113.540329]  ? try_to_wake_up+0x59/0x4b0
[97113.540331]  ? default_wake_function+0x12/0x20
[97113.540333]  ? __wake_up_common+0x72/0x130
[97113.540345]  i915_gem_execbuffer2+0x1b8/0x390 [i915]
[97113.540347]  ? __wake_up_common+0x73/0x130
[97113.540358]  ? i915_gem_execbuffer+0x2d0/0x2d0 [i915]
[97113.540389]  drm_ioctl_kernel+0x5f/0xb0 [drm]
[97113.540395]  drm_ioctl+0x31b/0x3d0 [drm]
[97113.540407]  ? i915_gem_execbuffer+0x2d0/0x2d0 [i915]
[97113.540411]  ? new_sync_write+0xe7/0x140
[97113.540413]  do_vfs_ioctl+0xa8/0x630
[97113.540415]  ? vfs_write+0x166/0x1a0
[97113.540417]  SyS_ioctl+0x79/0x90
[97113.540419]  do_syscall_64+0x73/0x130
[97113.540422]  entry_SYSCALL_64_after_hwframe+0x3d/0xa2
[97113.540424] RIP: 0033:0x7f5b11b6b5d7
[97113.540425] RSP: 002b:00007ffdc9f37c68 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
[97113.540426] RAX: ffffffffffffffda RBX: 000005ed1ee70030 RCX: 00007f5b11b6b5d7
[97113.540428] RDX: 00007ffdc9f37cc0 RSI: 0000000040406469 RDI: 000000000000000d
[97113.540429] RBP: 00007ffdc9f37cc0 R08: 000005ed1ea06500 R09: 00000000ffffffff
[97113.540430] R10: 00007ffdc9f37c90 R11: 0000000000000246 R12: 0000000040406469
[97113.540431] R13: 000000000000000d R14: ffffffffffffffff R15: 00007f5b070e8c00
[97113.540433] Code: 01 00 83 81 48 12 00 00 01 48 2b 05 6e 51 aa dd 48 c1 f8 06 48 c1 e0 0c 48 8d 04 d0 48 8b 56 10 48 03 05 67 51 aa dd 48 83 ca 03 <48> 89 10 83 a9 48 12 00 00 01 5d c3 0f 1f 44 00 00 66 2e 0f 1f 
[97113.540466] RIP: gen8_ppgtt_set_pde.isra.43+0x45/0x60 [i915] RSP: ffffa40fcce1b8d0
[97113.540468] ---[ end trace 9ac0a2bbf3104804 ]---