The beeping and blinking lights are indications of what the problem is. These vary between various computer manufacturers. A list of the more common ones is here: https://www.computerhope.com/beep.htm
One of the more common issues when attempting to restart a computer that hasn't been used in years is the failure of the "CMOS battery." See, e.g., the table for Dell computers at the above link: 5 beeps - CMOS battery failure. If that's the case, it's usually easy to fix by opening the computer and replacing the CR2032 battery (some laptops are more difficult). CR2032 batteries are widely available, including (in the U.S.) in CVS and similar chain "drug stores."