Bike won't shift at all I have old old 90's Cannondale that has been sitting in a spare bedroom for the last three years just collecting dust (has not been ridden in those three years). I bought a bike trainer over the weekend and so I wanted to throw that bike on the trainer and use it for indoor use.  

I set it all up and when I got on the bike and started shifting the bike shifting in the back to the largest cog (so lowest gear) but now I am unable to make it shift at all to a higher gear. Bike was shifting completely fine the last time it was ridden but now not at all. It also doesn't feel like there is any tension in the actual shifter when I push on it.  Does this type of problem sound familiar to anyone?

For what it's worth, the shifters are Shimano RSX.
I'm familiar with RSX brifters (brake and shift combo units)  If that's what you have then its likely the grease has gummed up, stopping the under-lever from engaging the release.

You might be able to finangle it a couple times by manipulating the underlever carefully, and you'll feel it catch.   A blast in the guts of the brifter with brake cleaner or similar can help short term too, but for the long term fix its at least an hour of messing around.

Here's a pre-existing answer where I wrote up the full procedure for cleaning a RSX brifter:

STI shifter sometimes doesn&#39;t catch in cooler weather

And here's the howto video showing... how to.

                
             

Answer: if a squirt of brake cleaner helps, then the full fix should make it right again.