The answer is simple...
If your the only one up and your teammates are dead theres a number of things that could cause it... (I will get into these later) but believe it or not L4D2 is a co-op game, and requires teamwork.

The most important, critical and best thing to do for your teammates is...
SHARE Now for the lone wolves out there, or just people who don't like to share might just disagree with me and just shrug off this guide with a dislike and a few hateful comments.

Then you will continue to fail missions, so i highly suggest you glue your butt to the seat and read... :P

Sharing like you learned in pre-school or kindergarten is different than this type of sharing, "Survival Sharing" if you will.

For example, if one or more of your teammates are low on health or on their last life, and you have a medkit you have a choice to make...

You can make the right choice no matter how new you are or your level of skill, think of it like this "I'm on full lives and i have close to full health, but this guy could use the health more than me." So you can give it to said person and heal them, not only making you a good player but increasing your team's chances of survival.

All you need to do is think, who needs it more.

Once you can master that, then your on your way to becoming a better player and not being a douche! Sharing = Survival.

Boosters, such as the pills or adrenaline.
They can help you, and your team in many sticky situations.

Such as Escape Parts in levels, where adrenaline could come in handy you can use them on yourself or give to a teammate, but these are more vital in situations than most know, so try not to waste them on a bad player unless you absolutely positively know that he or she is the right choice of survival.

So if you've mastered these... then your good to go.

Now slay some zombies, and survive!