How do you check the "vent?" (That was the next suggestion)
Take it apart and clean/check it. Once done and the dryer is running, start it and then check that the outside vent opens fully.

Can't get the back cover of the dryer off.
Here is a good site with troubleshooting and dis-assembly help. Note your unit is basically a GE.
http://www.applianceaid.com/dryers.html

Someone said it may be that we need a new thermostat, too. How do you replace them?
Just undo the wires to it and replace it.
Sounds like your thermal fuse may be blown.
Best and safest way to check most components is with the unit unplugged using a meter.