Nokia 7510 flashes 3 or 4 times when I turn it on, but it goes off again.

Recharge it. Is this in the house without any laser to activate the light/
If so - read the instruction again or seek a service center.

I've had this happen twice, both times when I tried to quit during a long web page download. Just taking the battery out briefly won't help though. It has to be out for somewhere between 12 and 24 hours. Probably there's a capacitor or a small battery that has to drain in order to reset some volatile memory.

SOLUTION FOR NOKIA 7510 (won't turn on/flashing black and white screen)

Remove the battery
Press the power button for few seconds
Install the battery back on
Turn the phone back on

it should work that how i did to my phone

forget my nokia 7510 master code

Possible that the software of the phone is corrupt or a possible hardware is defective.
You can remove the battery and memory card, have the card scanned for virus. replace battery after 15 minutes so that the phone is reset.
if the phone wont boot back the services of the Nokia support must be sought for reinstalling the software.

RPROGRAMMING YOUR PHONE MAY FIX THE PROBLEM.

My Dear Brother:
Flash The Phone With Latest softwear Version
Contact service problem will Quickly Solved

OR:
contact service center And Hilp the Service Oprator

flash the phone ( software problem )

all you need to do is to flash the phone ,your local phone repairs man can do that for you,but this is a temporary solution

Your software is corrupt. you must reprogram your phone.Reload its firmware.
regards