General ideas re storage: Ideal storage conditions for wine are cool, dark, moist, and vibration-free. Very dry air can compromise corks. Light and vibration are lesser evils, but still to be avoided. But the primary enemy of wine is heat.
Constant temperatures of below 48F/9C (i.e. Glamis Castle) will age wines at a glacial rate. It's unlikely that any backward wine would be ready in the original owner's lifetime.
Constant temperatures of 52F/11C will age wines very slowly but evenly.
Constant temperatures of 55F/13C will age wines evenly. This has become the de facto standard for most wine storage facilities and coolers.
Constant temperatures of 60F/15C will age wines faster, but reasonably harmoniously.
Constant temperatures of 65F/18C will age wines quickly, possibly with noticable loss of balance.
Constant temperatures of 70F/21C will age wines very quickly. Many reds will start to lose fruit well before tannins integrate. This temperature is probably fine for a few months, but dangerous for long term storage.
Constant temperatures of 75F/24C or above will lead to tired over-the-hill wines very quickly.
It is unlikely that exposure to temperatures of 75F/24C for a day or two would lead to any lasting damage. Once temperatures range over
80F/27C there is always the chance of a pressure change leading to cork failure and even if cork is sound, wines may exhibit signs of being cooked. This does NOT mean that taking a wine home in an unairconditioned car dooms it- it takes a while to shift bottle temperature.Many traditional passive cellars fluctuate seasonally- as long as swings are seasonal and not daily, this seems fine. Some argue that a wine that is in a passive cellar with a range of say 50F/10C in winter to 65F/18C in summer develop even more harmoniously than those in a constant 55F/13C controlled unit. But daily swings will probably compromise corks, and even if it doesn't will adversely affected the wine.
If one suspects that storage conditions are suspect, probably better to drink as soon as possible. For more information of temperature and aging, see this page: