Job alerts expire after one year. Once they expire, you will need to set up your job alerts again.
You can set them up by running a 'Test alert':
Which will then present you with the option to 'save alert':
You will also see that you can suspend or delete your alert.
It might take a day or so before you start to receive alert emails again. If you don't receive any after a week, please get in touch with our friendly member support team: firstname.lastname@example.org