Nothing. You do not win the item and nothing happens to your account. However, if the seller has trouble with the winning bidder and you are the next highest bidder, you could be given a Second Chance Offer to buy the item.
what do you think happens if no one bids on an item
You don't pay if you lose, so you didn't give any money.
The seller opens a 'dispute' with Ebay's resolution centre. Ebay will then contact the winning bidder to find out why they've not paid for the item. Once you place a bid - you are entering a legally bound contract.
Click the link to see how, under some circumstances, you can retract a bid on Ebay.
Ebay
Just bid on a item. You can pay for it if you won it.
In My Ebay under the Bidding section
NO. The horse is transferred to the highest bidder but you keep the money!
Ebay
No
yes
Yes, it is free to make bids on eBay.