I just took out the hard drive , but now the mac wont turn on and the sleep indicator keeps on flashing

The memory module nearest the Keyboard can sometimes pop loose especially when inserting or removing the module nearest the Bottom Case. After installing the memory I like to gently press on the edge of the module nearest the Keyboard using a non-metallic non-conductive item like a nylon stick or toothpick to make sure it is firmly latched. Just make sure not to touch any of the very small & fragile surface mount components on the memory or you will damage the memory.

You can also try disconnecting the battery and powering on the laptop using just the power adapter in case the battery is causing a problem. Try an SMC Reset.

Sure. Just keep an eye on the module nearest the keyboard to make sure it doesn't pop loose when reinstalling the other module nearest the Bottom Case.

If reseating the memory doesn't help, then you may need to try one module at a time by itself and test each slot to determine if one module or one slot may be bad.

blinking once, then repeating?

there is a possibility it RAM models were dislodged.