If your old contact lenses are over the expired time, you should not wear it. If your old contact lenses are still within the expired time but are in the solution which is not changed for a long time, you should not wear it. Or else, your eyes will be infected. Your eyes will feel hurt. In addition, your eyes will get pink eyes because of the invisible bacterium. You'd better take care of this and buy new type of the contact lenses.
Make sure your contacts are clean. If they have been well kept, I think it's ok to wear them. But if they haven't been used for a long time, there are risks for irritants and bacteria may have built up on them. So clean them thoroughly before you wear, or your eyes will be infected.
Even the contacts has been put in the solution all the time, it is not safe to wear them for the solution may lose its ability to disinfect the contacts. If you have no choice and you have to wear the old contacts, try getting some new solution to wash the contacts. If you don't have fresh solution, you can make some salty water. The salty water can kill most the bacteria, but not all of them and then clean them by distilled water. For safety's sake, you can use some anti-infection drops while wearing old contacts.
It depends on the time you haven't used your old contacts. If you haven't used them for a month or so, you'd better not use them. For the old contacts may not be comfortable as before. Generally speaking, people should change their contacts regularly. Firstly, due to the change of the eyes, the prescription of the contacts should be changed accordingly. Secondly, the state of contacts will change also. So if possible, youâ€™d better not wear your old contacts again.