After opening an .als project which has missing files, Ableton Live will notify you that is has missing files with an Orange banner located in the bottom left corner of the screen that reads, “Media files are missing. Please click here to learn more.” If you click the word “here” on that banner, Live will take you to the Missing Media Files option that is a panel that will appear on the right side of the screen. In the Search Folder section, specify the folder where you keep your samples or the folder where you believe your missing files to be located, then select Go in the Automatic Search section. If you specified the correct folder, in a few moments Live should find the missing files. Then hit save to update the Project on where those files are located so you don’t have to go through that process again.
To avoid doing all of that, you should get into the habit of selecting File, Collect All and Save, which creates copies of the sample files used to the project file folder so that your project is portable.