There is unlimited risk in taking a naked call option position. The only risk in taking a covered call position is that you will be required to sell your shares for less than the going market price.
I don't entirely agree with the accepted answer given here. You would not lose the amount you paid to buy the shares.
Naked Call Option
Suppose take a naked call option position by selling a call option. Since there is no limit on how high the price of the underlying share can go, you can be forced to either buy back the option at a very high price, or, in the case that the option is exercised, you can be force. to buy the underlying shares at a very high price and then sell them to the option holder at a very low price.
For example, suppose you sell an Apple call option with a strike price of $100 at a premium of $2.50, and for this you receive a payment of $250.
Now, if the price of Apple skyrockets to, say, $1000, then you would either have to buy back the option for about $90,000 = 100 x ($1000-$100), or, if the holder exercised the option, then you would need to buy 100 Apple shares at the market price of $1000 per share, costing you $100,000, and then sell them to the option holder at the strike price of $100 for $10,000 = 100 x $100. In either case, you would show a loss of $90,000 on the share transaction, which would be slightly offset by a $250 credit for the premium you received selling the call. There is no limit on the potential loss since there is no limit on how high the underlying share price can go.
Covered Call Option
Consider now the case of a covered call option. Since you hold the underlying shares, any loss you make on the option position would be "covered" by the profit you make on the underlying shares.
Again, suppose you own 100 Apple shares and sell a call option with a strike price of $100 at a premium of $2.50 to earn a payment of $250.
If the price of Apple skyrockets to $1000, then there are again two possible scenarios. One, you buy back the option at a premium of about $900 costing you $90,000. In order to cover this cost you would then sell your 100 Apple shares at the market price of $1000 per share to realise $100,000 = 100 x $1000. On the other hand, if your option is exercised, then you would deliver your 100 Apple shares to the option holder at the contracted strike price of $100 per share, thus receiving just $10,000 = 100 x $100. The only "loss" is that you have had to sell your shares for much less than the market price.