You are probably just signed into your other account and you can not be signed into 2 accounts at once.
Please follow these steps to get back to the other account -
1. Go to the inbox of the Gmail account
2. At the top right, please look for a square with 9 small squares, a bell and a circle.
3. Click on the circle and click "Sign Out"
4. Once you click "Sign Out" you will be out of the other account and you should see the account you want to sign into.
5. If you do not see it, please click the blue link that says "Sign in with a different account" and you can login to it.
If you have any questions, please do not hesitate to ask.