Can only select gears when engine is off.. Micra 1.4 2007?

2007 reg, nissan micra1.4 . Problem occurred suddenly. I find it difficult to engage any of the gears when engine is on; it is OK when the engine is off. When driving it is just about possible to get it in gear...but only just.. Is this an easy fix by a non techie, or a garage job and if so any idea of costs..thanks in advance for any help received...