Try to give as much info as possible. What were you doing last? Any strange behaviors before...
Whatever info you could give so that we can try to help you search a solution.

I flashed a rom to show my friend. After I was done showing him I went to flash the rom I was using before. It loaded up and started downloading my apps from the market automatically. It froze while downloading an app so I pulled the battery. Now it won't turn back on

So you get absolutely nothing? No lights, nothing?

Take out the battery and charge the phone its self without battery for a few minutes. After and hour or so of no battery plug them both in the charger and if it turns on wipe evey thing and flash previous rom. Don't know why but that happened to my cliq twice and it worked.

Yeah when I plug it in to charger nothing happens. No lights, nothing, not even the blinking notification light

Oh well I called Tmobile and they're sending me a replacement fit free on monday

Even though my phone will be here tomorrow, I want to point out that when I try to turn the messed up phone, sometimes the screen kinda turns on but just grey and turns off after about 10 seconds

That is interesting. I wonder if the battery burnt out...

Nope I put it in my wifes Mytouch and it worked fine

I have the same problem. My phone was frozen before I turned it off and now it off and does not turn on. I also tried plugging it in to the computer. It detects it but does not do anything. Im trying the charging the phone w/o the battery now. I hope it works. I've also tried going to recovery screen before white pressing the down volume button and power -- still NADA. Pls help. I may call tmobile to see if I can get a replacement as last resort.

I've tried the charging method w/o the battery. It does not work.

You may have to try to flash the firmware from RSD Lite

I thought RSD lite was only for motorola devices?

Omg I'm sorry. Of course it is.

I have the same problem with my lg P350 it crashed from a rom and it went recovery mode so i downloaded android firmware again and later found out i downloaded the firmware for P500 now it wont turn on at all even when plugged in nothing shows up.. is there anyway i can fix this?

Can it boot into recovery?