arduino nano 5v pin drop

I measure my arduino nano between the 5v pin and ground it has 2.8 -3V drawn,, I measure the other nano and it has 4.8- 4.9V what should i do?it cannot power up my relay module

If you have 2 different Nanos and they are reading differently by a significant amount when hooked up the same way, one is probably bad. You would have to troubleshoot the bad one at the component level, or get a fresh Nano.