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 "Me" and then tap "My 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? Now you can! Go to "Me" from your bottom menu bar > select "My Numbers" > Select your number and scroll down to view all your options.