A sudden loss of mobile service can have many causes, but an unexpected SIM replacement is one possibility when it happens alongside unusual account activity.
Contact your mobile network provider through an official channel and ask whether a SIM replacement or other change was recently made to your number. Explain that you suspect an unauthorized SIM swap.
Secure important accounts, especially email, banking, and messaging accounts. If possible, use an authenticator app or another security method instead of relying only on SMS codes.
Check banking and other sensitive accounts for password resets, new devices, transfers, or other activity you do not recognize.
If your WhatsApp or another account has also been taken over, use the relevant incident guidance for that compromise.