Comment 1 for bug 1551115

I've noticed couple of similar reports for some older releases, but none of the proposed solutions worked for me:
http://askubuntu.com/questions/459720/14-04-password-not-accepted-after-computer-wakes-up-from-sleep

I did not have such issues with 15.04.

$ sudo lshw -c video
  *-display
       description: VGA compatible controller
       product: Seymour [Radeon HD 6400M/7400M Series]
       vendor: Advanced Micro Devices, Inc. [AMD/ATI]
       physical id: 0
       bus info: pci@0000:01:00.0
       version: 00
       width: 64 bits
       clock: 33MHz
       capabilities: pm pciexpress msi vga_controller bus_master cap_list rom
       configuration: driver=radeon latency=0
       resources: irq:32 memory:c0000000-c7ffffff memory:cc400000-cc41ffff ioport:3000(size=256) memory:cc440000-cc45ffff