I'm having a concert tomorrow and was rehearsing today, my acoustic guitar fell to the floor 'cause the strap broke. When I picked it up, at first sight wasn't broken, but when I played, the fifth string in the fifth fret, doesn't keep the note, it is played just for a second, with a metalish sound, It's the only string that does that, even on other frets, the guitar plays as it should. How can I fix this? Thanks
It sounds like you may have popped a fret, although without pictures or more investigation it is impossible to tell. The sixth fret may have come out of its slot a little bit and is now higher than it should be, causing the string to hit it when played on the 5th fret. It is possible to tap the fret(s) back into place, but unless you identify the high frets for sure I don't suggest attempting it.
You could try loosening your truss rod a little to give you more relief on the neck, raising the action and possibly clearing the high fret. Again, don't try it if it is something you haven't done before or are not comfortable making modifications to your guitar.
It sounds like there isn't much real damage to the instrument, so a guitar technician should be able to have the problem fixed very quickly. It may be worth calling around and finding someone available to look at it before the gig.