Had this happen to my SNav about 2-3 years ago and when I was ready to go
X/c so I was screwed.

Another pilot who had previously had the same problem leaned in to the back
of the panel, put his hand in near the back of the Snav and give it a thump
with the side of his hand. All went back to normal and never had the
problem since.

Hope this helps.

Andy