Whether you should try to repair the old one or buy a new one depends on a bunch of things. What is wrong with the old one, can you even get parts for it, and if you can, what do the parts cost? What is the wattage of the old heater and what will a new heater of the same wattage cost?
Some of the new baseboard heaters are pretty inexpensive, so I would compare the repair costs to what it would cost to buy a new one. If the cost of the new heater is even half-way close to the cost of repair for the old one, I would go with a new one. I mean, I am guessing you have had this heater for a long time and therefore you have gotten a lot of use from it. Rather than start spending money on the old one now, spend the money you would have spent fixing the old one on the new one. You will probably have to add some money to it to in order come up to the price of the new one, but so what? You will have a brand new heater with a warranty.