This was used for 6 mths and has sit for 3 1/2 yrs. I have connected the mini USB to charging cable that works fine on my phone. Nothing happens, no green light, no anything and it does nothing when power button is pushed for up to 30 seconds. It came with an external keyboard. Does it need to be connected?
The battery is likely toast. Sitting for that long could easily lead to a dead (not out of power) battery. Additionally, some cheaper devices will not work after a time of being off. And off for that long would not shock me.
You can try replacing the battery and see if that resolves it. If the tablet has a hard to get to battery, then I would suggest looking online for walkthroughs (video or text) on how to access that particular devices battery, or take it to a local tech. Otherwise, depending on the cost to repair, it might be cheaper to get a new device.