My Interpretation: You changed your strings and your bridge is lifting off the body of the guitar....
Causes: 1. Since you probably don't know much about the guitar you probably didn't account for stretching the strings as being part of the string changing process... Strings are not pre-loaded so you have to stretch them to release any stored energy within the string (excuse my scientific blabber).
2. Another possibility is that you tuned your strings way too high... and they are pulling harder on the bridge than what they should be... causing it to lift off the body...
3. The last possibilty is that you switched to a higher gauge of strings than what your guitar is set-up for... Causing you to put more tension on the brdige than what it's set-up for.
4. If your guitar is acoustic and the bridge is lifting.... the causes can come from extreme temperatire exposure or extreme humidty expouse... the glue has either melted or dried out completly and became brittle causing the string tension to pull the bridge away from the body....BAD NEWS
Solutions:
1. Stretch the strings by pulling up and down lightly toward the middle of the string... then retune your guitar... stretch them again... then retune your guitar one more time. If your bridge didn't fall back to the point that it was at then stretching the strings wasn't your problem... however, you still probably needed to do it.
2. You went to tune your guitar and you started cranking on the tuners and you went way too high. If you do this... in most cases, the e, B, and G strings will probably just break... The E, A, and D strings will take a lot more punishment when it comes to over tuning... if you can bend the strings easlily this isn't your problem... if you can't bend the string hardly at all... you need to tune your strings way down becuase you went too far.
3. This is likely... when you bought your guitar it was probably set-up with either 8's or 9's... thats the average string gauge for factory set-ups. If you bought a set of strings that are higher than this it will cause you bridge to lift. If this is the case... you will have to set-up your bridge to accomodate the higher string tension needed to keep these higher gauged strings in tune.
4. If this is the case.... stop what you are doing NOW! relieve the string tension... and take this guitar to a guitar shop and have the bridge reglued.
I hope one of these scenarios can help you solve your problem... and in the future, if you can give more information about what type of guitar, what type of bridge, what gauge of strings, so on and so forth... that would help all of us out a tremendous amount. Good Luck