Sorry that you lost your TextMe number. Numbers are reassigned when not used for a certain period of time.
You should have received a warning notification to invite you to use your number before we take it back.
Unfortunately, you cannot get it back but you can request a new one.
For that, from the bottom menu, tap "Numbers" and then "Get a new phone number".
In order to prevent losing it, please make sure you use it at least once in the first 7 days after your signup (3 days if you have a UK or a Canadian number) and then, once every 20 days for US number (once every 10 days for Canadian and once every 15 days for UK numbers).
Not using TextMe frequently but want to keep your number nevertheless? You can now purchase a subscription to your number. Tap "Numbers" to see your options :)