Hi Craig,
In the Try out Git exercise you should've activated a sandbox which expires after 2 hours. If you are in a later unit and you want to access cloud shell you can open up a separate browser tab and navigate to https://shell.azure.com . In upper right corner it should have your account with MICROSOFT LEARN SANDBOX underneath.
You can switch back and forth between the tab with Azure Cloud Shell and the other tab which has commands that you can enter. Please complete exercises in order, and if your sandbox expires go back to previous exercise where you first activated sandbox and active a new one. Once activated, re-complete the exercises in order.
Please click Accept Answer and upvote if the above was helpful. If something is unclear or you still need assistance, add a comment below.
Thanks.
-TP