If you mean the plastic overflow tank, replace it. If you mean the plastic tanks on either side of the radiator core, replace the radiator.
Replace it with a new or good used one.
Best bet is to replace the radiator.
blown head gasket or cracked head.
if its in the reservoir it should be in the radiator of it is then it has a blow head gasket or a crack head
Is it leaking from the side by the coolant reservoir?
No, this could mean you have a cracked head gasket and engine pressure has entered the coolant.
A cracked radiator can not be fixed. A cracked radiator needs to be replaced . A mechanic can replace one in a few hours.
The only way to fix that problem is to buy a new radiator. Your old one has cracked around the fitting.
You can replace pipes and hoses relatively easily, but if the radiator itself is damaged, you can't fix it. You will need a machine shop that can actually repair the cracked metal.
Both. The reservoir is the overflow. But in the event your radiator is low, your reservoir will back it up.
You need to but a new one soon or your car will overheat from lack of pressure to keep the water cooled. Compared to most car parts they are inexpensive.
If it's repairable, a reputable radiator shop will be a good resource. They can tell you if the radiator can be repaired or if it must be replaced.