Originally Answered: Why can't I sing high notes?
Either you are about to catch a cold, are dehydrated, are not warming up your voice properly, are overdone your voice (in terms of singing time and/or vocal range) or have incorrect, unhealthy singing and/or breathing techniques in use.
It takes obviously some talent, LOTS of patience, diligence, courage, hard work, dedication and LOTS of lessons with a GOOD vocal teacher. Besides, your voice will keep maturing until you are in your mid-30s so you need a lot of TIME as well. Therefore:
THE ONLY SAFE way to learn the correct singing techniques & to improve properly IS to take OFFLINE face-to-face singing lessons with a fully trained vocal teacher (in case you do not have a teacher yet)! The teacher HAS TO BE in the same room with you, so that he/she could give you proper feedback. However, even the best teachers in the world cannot make wonders, so please be realistic with this. Singing lessons are NOT going to help if one is tone-deaf!
Please do NOT rely on any dodgy web tutorials because that way you can misunderstand things VERY EASILY and develop bad habits, hoarseness, vocal nodules and other nasties IN NO TIME, and even though you would sound good! It is always much wiser to invest a little bit of your money/time to face-to-face lessons rather than wasting the same amount of money (or even more!) to frequent ear-nose-throat specialist visits due to aforementioned problems, so please reconsider this. If you can't afford vocal lessons, then joining a choir is the only SAFE alternative option. And believe me, but even MANY of those who have music as their hobby DO take lessons as well!
Always remember to warm up your voice properly, but please know your limits and don't overdo your voice! Remember the diaphragmatic support, do not strain your throat too much! Also, remember good body posture!
Avoid fizzy drinks (burp danger), dairy products (mucus risk), caffeinated products (coffee & tea included, they dry up your throat) and spicy food (irritation risk)! You can consume these things, but NEVER before singing!
Do NOT shout, yell, scream nor otherwise abuse your voice AT ANY TIME! Also, please respect your vocal range; if your teacher says you are, say, more of an alto (baritone if you are male), then you ARE more of an alto (baritone).
DO NOT try to imitate anyone famous, that will usually give you just bad habits and even damage your throat. You are YOU and your voice is unique, so please learn to cherish that. The world does NOT need copycats.
Do NOT sing, whisper, shout, yell nor scream if having a sore throat/cold/flu, Also, do speak as little as you can if you have flu/cold/sore throat!
Remember to drink at least 2 litres of room-temperature still water every day, not just during singing days!
Smoking is a big no-no, as is inhaling secondhand smoke. Also, avoid staying in dusty and/or moldy environment.
If there is ANY pain/discomfort present, STOP immediately and go see an ear-nose-throat specialist in order to rule out vocal nodules and other nasties.