Time Limitations Edit
You can only enter a dungeon once per dungeon lockout. Each dungeon has separate times that it has, generally increasing as players level up. While locked out of a dungeon it is not possible to enter that specific dungeon, you can however enter a different dungeon while you wait. As a bonus when using the Instance Matchmaking tool, the dungeon lockout timer will be halved.
Level Limitations Edit
Instances are designed for a certain character level - mainly because of the level of the mobs inside. Higher level mobs will crit your tank more often. Lower level mobs, on the other hand, may not pose enough of a challenge and will give you little to no experience.
The Instance Matchmaking tool shows the instances suggested for that level.
Each instance also has a minimum required level to actually be able to zone in, however this is considerably lower than the suggested level. Both these level requirements can be checked at the Instances by level article.