Jquery conflict with my custom theme

After installed this plugin am getting Jquery error something like TypeError: $(...).leanModal is not a function so all the following scripts used on my website are not working, even the Private messaging functions are not working

i can't see the file attach popup window, Send to auto suggestion list. i thought this happen about the Jquery conflict , is there any possible way to fix this issue without over ride the core files of Private messaging plugin ??