Comment 37 for bug 797922

You might check that with nomodeset whether resume really hung or the display just didn't come on. I.e. ping the device on the network, or install an ssh server and try to log in.

Here's the link I forgot to include earlier for the systemtap debugging. Since it doesn't look like you have any other way to get output during these hangs, this is probably the next thing to try.

https://wiki.canonical.com/PlatformServices/HardwareEnablement/Documentation/PowerManagement/DebugTools