It's probably the waste ink protection counter kicking in.
Inkjet printers produce a small amount of surplus ink with each operation, which is collected internally. To ensure that it doesn't overflow, printers are designed to stop working (so that the waste ink can be removed) after a set number of operations. Emptying the waste ink tank is theoretically a job which can only be done by the manufacturer (probably at a greater cost than that of a new printer!) but you can sometimes find d-i-y instructions on Youtube.
However the counter usually kicks in well before the waste ink tank is anywhere near to full, so you simply need to reset the protection counter. (Remember though that there's a consequent risk that at some time, probably well into the future, the tank could eventually overflow).
You can reset the protection counter with this free program:
http://www.ssclg.com/epsone.shtml
Chris