I just use web checkin for my flight tomorrow. After that I should have printed my boarding pass. However, I realised that this is not possible. So what can I do now? I have already checked in, but no boarding pass. Is there any way to print that again at the airport? Or is it enough if I have my confirmation number? Or the boarding pass as a PDF?
Many of the major airlines do have self-service checkin machines at the airport. I know KLM, allows printing forgotten or failed prints.
If your airlines does not have these self service machines, and you are not yet at the airport, try going to a print or copyshop. Most airports these days have these shops, but sometimes they can have quite some waiting queues, which will be adding stress to your journey.
Depending on the airline you are traveling, you could choose to use a mobile device as an alternative boarding card. This only works if your boarding pass has a bar code they use to scan prior to boarding. What you need to do is make a pdf of your boarding pass and store it on your device. I always do this these days and this already saved me once.
A final option is to just approach the check in desk and ask a reprint. This is the least favorable solution, since the whole idea behind online check in is to minimize contact time at a manned checkin desk. Some airlines are then reluctant to help you in these request, some airlines (Ryanair comes to mind) have a hefty fee if you ask for a reprint. In the case of Ryanair this is 40 euros or even higher.
I fly Air Canada as my primary airline. It is worth checking in online even if you don't have a printer available. There are no negative consequences compared to not checking in at all. You can line up to see someone and hand off your baggage, and they'll "reprint" your boarding pass, or you can use the kiosks (I have never seen a lineup for AC kiosks, unlike United for example) and go through the checkin process again. You will retain your original checkin time should that become relevant for things like processing upgrades or deciding who loses their seat on an equipment change. (See my answer to "Benefits of checking in online when checking bags".)
You can reprint it at the airport, no problem. If no self-service station is available, the regular check-in desk personnel will reprint it for you.
Some airlines in some locations allow using "mobile" boarding pass through their smartphone app, if this is the case for your airline and airport - then you don't even need to reprint anything at all.
Don't know what airlines you're referring to....but visiting a manned check-in desk is definitely the preferred option for MOST passengers. (Note: ryanair is an exception - all other airlines will allow you to seek help from a customer service representative).
Flight bookings these days can be complicated, and it's always best to seek assistance from the airline staff. Self-service compliments the whole check-in process, but should never be a complete substitute for dealing with airline staff.