On this page of the guide to Back 4 Blood, you will find out how can your character die and what will happen when everyone else in the team also dies.
Death of your character
When the health bar of the character you control reaches zero, the character will fall to the ground, but not die. At this point, a new health bar will appear, this time separating your from actual death. When you are knocked down, you can use secondary weapons that you have equipped, e.g. a pistol, a shotgun or even a melee weapon.
You can be revived by your comrades, but if they do not do it in time, your character will die. Then, you can be only revived with a defibrillator. If someone does not have a defibrillator, then you can take control of a bot, but only if your squad uses such an NPC bot.
When it comes to what you can die from, there are many options. The most obvious are zombies, however, you can also die from explosions, falling off the map, and even from allies if friendly fire is enabled.
Death of the whole team
If your entire team dies and you fail to complete a given location, you lose your number of extra lives. They depend on the difficulty level you have chosen and the cards you have selected. After losing, where all the players died, you have to start from scratch.