Without knowing what is missing or the type of AC system you have we cannot give you an answer to your question. A few photos of the unit and the part that is missing or where it fell off can help a lot.
More information is needed. What type of unit is it and how do you know that a piece is missing. Pictures would be wonderful
You may be able to fix your airconditioner on your own. However, it is better to call aProfessional HVAC expert. Hiring a pro for AC installation is a better optionif you are not technically equipped to do the same as it in involves manytechnicalities to perform the activity. You can perform some maintenance tasksyourself, but a professional on both your air conditioning and heating systemsonce per year is required a for a thorough tune-up.