naturally the first step is to move the mouse and/or press spacebar on keyboard a few times. If no joy, some computers require a quick press on the power button to wake it up - but be careful if you have unsaved data, as this might actually turn it off.
if the computer refuses to wake up, you will have to hold the power button down until the unit powers off, but pay attention to loss of unsaved data as stated above.
if this is a recurring problem, you should look into updating the BIOS from the manufacturers website, if it isnt already the latest.
if that doesnt solve it long term, you will have to do some APM troubleshooting steps, which we can follow up on later.
also, some misbehaving software could be causing the system to be unstable before suspending, so in the future i would look in the FAQ area on steps to remove adware and optimize your system, to fix disk errors, etc. also remove any "junk" programs you dont need.
the idea being, the more clean and lean the system is before suspending, the more likely it it to wake up happily.