Does anyone have some idea on my A/C repair?

[casliupeng]

Hi everyone,
I checked my A/C from a repair store several days ago, and the maintenance guy told me that the hose was unplug by somebody and he did not know why the personal did this. What the maintenance guy recommend is to replug a new hose and replace the condenser, and then do the check to see which part is not working. However, the fees are about 600 bucks so I decide to repair by my own. Does anyone have some experience in checking A/C before? Could you give me some advice (Including which tools I need to check A/C, what procedures for check, what need I pay attention)? Thanks!