Did they work before? Do you still have some text allowance left? Do all texts fail to send or only some? If only some then what sort of numbers are you trying to send to? What error do you get when trying to send texts that fail? If necessary you might want to contact 48 support. See the link in my signature below.
Hope this helps.
I am not a 48 employee so if you need to contact 48 support then use option 3 or the chat facility here.