If TracerPlus was recently upgraded on your device (to 10.5, for example) and upon the next re-open of the application you are prompted to Login, Register, or Try It. You will want to re-enter the appropriate information to get back into TracerPlus.
As a subscribed user you will need to select the Login menu option. This will allow you to re-login using your Subscription credentials. There are two ways to login, the first is with a username and password, the second is with the Device ID. Depending on how your account is configured you will need to login slightly differently.
If you have a username and password, you will need to enter a valid subscription username and password for your account and click Login.
For users with accounts that login with Device ID, click the hamburger menu on the login form. Then make sure the Login With Device ID toggle switch is enabled. Click Done, and enter your Account ID, then click Login. In rare cases that your device ID has changed, you will need to login to the subscription portal at https://subscriptions.tracerplus.com and disable the previous ID for this device in order to allow the changed Device ID to login.