My sony ericsson k800i just fall down and it wont turn on its only the infrared blinking

I have an Sony Ericsson K800i, I had the same problem with that phone and i ended up with knowledge on how to fix it.

If the phone does not start and the only thing functioning is the red LED light at the infrared panel, this means that somethings wrong with the EROM.

EROM is like the boot up code on a computer.
It seems the EROM has been corrupted in a way or another. This is a very common problem with K800i's.

The best way to fix this by using a flashing program such as ("SETools2Lite") This has the capability to flash your phone's erom, including your other parts aswell. All tho for flashing your phone completly you will eventually need the right files for your phones.

ALSO a MAJOR TIP: If you use these programs, i STRONGLY recommend that you DO NOT in what case it might be EVER to Upgrade your phone to a CID:53

If you ever se the word CID:53 just avoid it. CID:53 could mean death to your phone for all eternity if your not willing to pay to get it fixed.

Little details about CID, Its functions is to keep people from tampering with their phones such as putting K810i in a K800i.
Re-flashing and fixing your phone by yourself. CID:53 is the latest of the CID's for these phones. CID 53 is like an IRON CURTAIN while the rest below 53's are more like a normal curtain.

For guidance and assistance on fixing any Sony Ericsson phone i would like to recomend you go to http://www.topsony.com/ - Register ( which is free ) and go to the forums with your problems.