No, it is not safe to wear the old contact lenses because there exists the expired time for the contact lenses. When it reaches the using time, the function of water content the contact lenses own will lose. If you insist on using it, your eyes may be dry which easily get infection.
I not recommend you wear old contact lenses. You know, there are lifecycle for contact lenses. So, you shall check if the contact lenses expired. Besides, as we know, there are weekly contact lenses, monthly contact lenses etc. Which one are your old contact lenses belongs to. Are they overdue since you open it. If the contact lenses overdue, you'd better not wear it. Or you may get eye infections.
Do not do that! It is very dangerous! Cause it will damage your cornea seriously.
Under normal circumstances, doctor would like to advise you to replace your contacts regularly. There is service lifetime for each contacts. And your eye's situation must have been changed, like myopia degree increasing.
You should get a pair of glasses as spares, because the period of wearing contacts do not exceed 12 hours every day based on the doctor's advice. And 8 hours should be better.
If you do not have glasses, the only way is to wear the old one. But that is expedient. There are many virus and bacteria on the old one. Clean it thoroughly before wear.
Rush to buy new contacts as quickly as you can!