These aren't infallible, but you might like to give them a try:
1. Hold it under the hot tap for a few seconds.
2. Jab the ballpoint hard several times on some card or other substantial writing surface. The ball might have seized up, and just needs a bit of 'persuading'.
These assume that there is still some ink in the pen.
Go visit your local stationers or maybe a newsagent. Buy a ball point refill. Open you pen and dispose of the frustrating refill that has seized. Replace with a brand new refill and screw the pen up again. Try a few scribbles on a spare bit of paper in order to get it started.
Don't buy biros at the Pound shop - unless you keep the receipt. I bought 20 and very few worked.
Try warming the first inch in water.
incidentally a printer cartridge which had not been used for a long time failed so I wrapped it in clingfilm and tucked it between my thighs in bed for half and hour. Success!
For writing a letter I like to use a fountain pen as I've found since using the word processor my hand writing is no better than a scribble, I use ball points for crosswords and little memos to myself (I'm a tad forgetful) in reply to the main question, I lick the top of the ball point when it seizes up.