meaning its out of battery now?

if yes, try to get the battery charged using a "external clip-on charger" which you can charge the battery using the 2 clips of the external charger
it can be bought for abt rm10 fr most fon shops, the charging takes abt 2~3 hrs, and it is a slow charge which is a VERY good alternative, i used tat to charge my spare battery and it can last LONGER than a normal charge via the fon too

when flashing rom, need to take out sim card and sd card or not ?

no need~

then is it a must to turn off the phone while doing the rom flashing  ?