How to safely start my iMac after years?

There isn't any reason to suspect it won't start. If its NVRAM battery is dead (likely, as rkaufmann87 wrote) it should still be able to find its startup disk and boot from it, but you will want to replace that battery at some point.

Fortunately that is one of the easiest tasks to do on that specific model Mac. You don't even have to remove the display assembly — you just need to open the front bezel enough to get to the battery. As far as iMacs go, that operation is as easy as it gets.

A dead NVRAM battery means its real time clock is almost certainly wrong, which can wreak all sorts of havoc. Setting it should be the first order of business. You may need to find its original, grey, System Install DVD so that you can boot from it and use Terminal to reset the clock. You have that disc, right?

Almost for sure the PRAM battery is dead, change before attempting startup

If it was working and stored properly, then it will either startup or show one of the following screens.

If it doesn't startup all the way, see > If your Mac doesn't start up all the way - Apple Support

You're welcome.

Apple no longer has or supplies any of those old original Install Disc sets.

If it will not startup and you can not find the original gray Leopard Install 2 disc set in your collection, then look for a Retail Snow Leopard Upgrade DVD or the Lion Upgrade on a Flash Drive.

Don't do anything about the condensation. As long as you continue to keep at it room temperature (not in cold storage) it should be fine.

Your welcome, regarding the condensation the best thing you can do is to leave let the computer run 24/7 to let the condensation dissipate.

Miggory wrote:

i had my iMac in storage for 4 years, took it out last year, it worked fine. Then packed and left for another year, now it won’t start. What do I do? 😩

I would suggest you start your own thread with your issue rather than tagging into someone else's question. It's more polite to the original poster, and is more likely to get you a quick and helpful reply. When you repost, please include full details about your issue so people have the information they need to be able to offer a suggestion.

Regards.

Miggory wrote:

i had my iMac in storage for 4 years, took it out last year, it worked fine. Then packed and left for another year, now it won’t start. What do I do? 😩

If it won't turn on..? see > If your Mac doesn't turn on - Apple Support

If it won't startup..? see > If your Mac doesn't start up all the way - Apple Support

Your welcome and good luck.

i had my iMac in storage for 4 years, took it out last year, it worked fine. Then packed and left for another year, now it won’t start. What do I do? 😩