Tom,
If you can provide any more info on what sketch you may have loaded - ie was it an example, was it a Digispark example or a stock Arduino example, etc, etc I'd like to make sure it wasn't related to a sketch somehow corrupting the bootloader - though I really doubt it as I've tried to do that and haven't been able to.
Otherwise it sounds like it might just be faulty - send me an email and we'll take care of you. support at digistump dot com