Serial column has suddenly stopped working

I have been using a serial column in my most important postgres table
for a couple of years.

But today it has suddenly started assigning zero instead of the next
number, which clearly is causing chaos.
I have restarted Postgres, but it has not solved the problem. I cannot
re-assign the serial number because it is tied to other tables.
Anybody had this problem??

I have reached number 14954 which we can't exactly call a high number.

The column description is as follows:

Name srl
type bigint
modifiers default nextval(booking_srl_seq::text)

Any help greatly appreciated.