If your account is still active, nothing needs to be done to re-set the exams. You can just keep clicking the "take a CAT" button to start a new test.
If you can no longer access the tests, that may be because your account access has expired (or there may be some other tech problem). Contact our Student Services team at 800.576.GMAT (US and Canada) or
gmat@manhattanprep.com and they'll be happy to help you figure out what's going on.