if "off" means powered down, the messages should come through when it's powered back on again. Though, I use the word "should" because messages are queued for a specific period of time before they're dropped. That time could be a day, it could be a week, I do not know how long it is.
If "off" means you didn't pay your bill yet, I'd say it's a toss up of the above or, they're just dropped because the account is suspended.
Manage your T-Mobile account