Shutdown when i turn on my mp3 player it says building database then goes to shutdown battery is fully charged

The Batteries in these units like all rechargeable batteries only have a certain number of times they can be charged and discharged before begining to fail. Your only option is to replace the battery with a new one. They are cheap to purchase off eBay and YouTube has Videos detailing how to change them.

my rca player won't turn on. it looks like it is going to [screen ilghts up] but then it goes black again. the battery is fully charged

maybe you locked the key behind mp3 player. or it does not have enough chaging

Insert a microSD in the slot and then try turning it on....

Here is the user manual - it will show you a picture of where the hold switch is:
http://www.rcaopal.com/pdf/M40xx_IB.pdf

I have an opal also and it sounds like yours is defective.You should try returneing it to the factory if it is still under warrenty.

I would send it back to the RCA company because it sounds like it has a short in the system.

leave it on till the battery runs out or push the reset button on the back of the player with a pin, or a paperclip

There is something with your device firmware..get a firmware upgrade done in your device..that should solve the problem...

Have you tried to remove the Battery and put it back again?