For some reason the vnav button won't work.Any ideas why?.RegardsMartyn

What phase of flight? Is your FMC programmed? Is your MCP set properly? Hmmmmm, can't think of any more questions besides, can you provide more details so that we can help you?

Thanks Fred for your responce, here are the details.FMC programmed for flight level F220, 300kt over about10 legs(320 miles). Set MCP level to F220 and speed to 300kt. Take offand at 2000ft press autopilot, autothrottle, fnav which works,vnav which doesn't.Cannot understand, it was alright a few days ago.Martyn.

When you're trying to engage VNAV, what does the FMC say?