2001 GMC Sierra Unable to Go Forward to Reverse

Because your Sierra just stopped, it is probably an electronic problem. If the problem were mechanical, then your pickup would still be moving, albeit haltingly because the transmission would be slipping Or the tranny would be slamming between gears, while your pickup would still be performing poorly. In either case, though, notice that the pickup is likely to be moving, not stopped.

If, however, the transmission control module had failed, then your pickup would have performed just as you describe: it would have stopped. Then, when you went to start up again, the pickup would have started and it is likely that you would have heard the transmission trying to work with gears trying to engage, just as you describe.

Don’t worry because the transmission control module, while it is mounted on the valve body, which means accessing the device will require a partial teardown, is a relatively inexpensive device, as transmission parts go. It will likely cost you about $400 total for the work. The TCM needs to be replaced, possibly the solenoids the work with it, too.