In his fantastic book Enchantment, Guy Kawasaki says "When people contact you, respond fast... in less than a day. Few people respond quickly... Luckily for you, this means a fast responder is unusual and therefore more enchanting." This is great advice for a competitive field like illustration. Unfortunately, I am often guilty of the opposite. I mull over a response, wait a day or waste time double checking for typos. This habit is a handicap.
The most important thing a freelancer needs is immediacy. The ability to respond immediately and actively engage an opportunity should one arise. No matter how important it is to also be polite, hardworking and always finished before a deadline, if you don't reply fast enough you don't have the job. Period.
I received an email at 3 PM on a Wednesday for an ad design job. In the middle of a family emergency, I did not read the email until the following afternoon. I called the employer, but it was too late the job was filled.
The past three weeks I've been sick with sinus infections, pink eye and (I'm not kidding) Montezuma's Revenge. To top things off I lost my insurance coverage, had to pay high income taxes and was summoned to jury duty. During this time my greatest desired internship was posted. I worked on my portfolio but unsurprisingly couldn't focus. I didn't apply in time and so wasn't considered.
If you can't turn up, it doesn't matter what you went through or how great you would be for the job. The best you can do when a ship has sailed is be aware, don't get discouraged and be ready the next time around. Employer mentioned in the first example, actually hired me for the rest of the advertisement campaign. Fheew! It pays to follow up. There will be other internships and other jobs. Be ready for them. And read Enchantment, it's excellent.