I have a lenovo ideacentre 300 desktop which i have bought bew ram and installed it but i have no picture and 3 short beeps and one long one?

New RAM has to be compatible with the machine. Go to where you can find out what you should have bought. Put your old memory back in of course.

You have either not plugged the RAM back in properly or
not bought the correct RAM or
damaged something or
The new RAM is faulty.
Plug the old RAM back in and let us know the result

You bought the wrong type of RAM.
next time ask a Computer Shop for advice first.

3 short beeps and 1 long beep is memory error. Something wrong with RAM you replaced.

I have bought the new ram through crucial and they recommended the ram and its correct. It won't even work with the old ram now and i have checked its properly clicked in

Do a search for LENOVA IDEA CENTER 300 BEEP CODE
BUT IT SOUNDS LIKE YOU INSTALLED THE RAM MEMORY WRONG, 1) WRONG KIND 2) WRONG SLOTS 3 ) NOT CONNECTED FIRMLY IN SLOTS (oops I wasn't yelling)