First off don't worry, 99% of the time the checklist just wasn't submitted.
If you are on the same device that completed the checklist, then it was saved locally on that device. In addition, if you had WIFI when you completed the checklist, then it is also saved on our servers.
First, check the Completed Report. Make sure you use filters for the location and the checklist to narrow the list down. Also, make sure you are in the correct child org, if your company uses orgs.
If you don't see a completed checklist in the Completed Report, then select the location and navigate to the checklist. A local copy should be there and you just need to scroll to the bottom and hit done and let it submit.
If you are on a different device, then log in and navigate to the location and the checklist and when you open it up, you should see a menu similar to this:
The image above is showing an in-progress checklist. You should see your name, then just click the button and the checklist will download to the device you are on.
Once in the checklist, hit Done at the bottom to submit it.