Mobile data stopped working, wifi fine

android or iphone? either way, start with the dull stuff: airplane mode on and off, a restart, and check you haven't hit your data limit, a lot of plans just stop data when you do. on a dual sim phone make sure the right sim is set for mobile data. if that's all fine, reset network settings (it forgets saved wifi passwords, so have those handy). on android also look at the APN settings under mobile networks, if they got wiped or changed, calls and texts work but data doesn't, which matches what you describe. your carrier can tell you the right values.