|Make sure the disk is not dirty or excessively scratched. If it is dirty, wipe it off radially, not in a circle.|
Clean the laser lens on the drive - use a laser lens cleaning CD, or on laptops, usually you can easily clean it when you eject the tray, when there's no disk on it.
Not all media works fine in all optical drives.
All optical (CD and DVD) drive models are a little (e.g. LG) or a lot (e.g. Pioneer ) sensitive as to which media - brands and types of CDs and/or DVDs - will work properly with them. The manufacturer of the drive usually has info about that in the information for the model on their web site, or if you bought the drive yourself, there is often info about that included with the drive when you buy it.
E.g. LG drives recognize many types of media, but the only brand they specify as working fine with all types of burnable disks their DVD combo drives (read and burn both CDs and DVDs) can use is Verbatim.
Your optical drive must be capable of reading the type of disk you're trying to read, and if it is a burned disk that already has data on it, it must be able to read it the way it was recordered. The types of disks a model can read and burn etc. is specified in the specs for the model. e.g. Some burner drives can read RAM or Mount Rainier recorded burned disks, some cannot.
Of course, you can't read a DVD on a CDrom or CD burner (only) drive.
Burner drives are not 100% compatible regarding the way they record.
You should be able to read a factory ("stamped") original CD or DVD, or CD-R or DVD-R burned disk on any computer, but another type of burned disk may not read properly in a drive it was not made in, even when the media type is supposed to work fine in the drive model you're reading it in.
"Scratched disk? CD-RW? My LG drives seem to hate really old (before 2000) CD-RW discs..."
I have seen in reviews on the web that t=est the drives that the one weakness LG drives in general have is they are not as good at reading scratched disks as some others.
I have also seen where R-W disks that used to read fine in a particular model do not read fine as time goes by. Going by that I wouldn't put important data on RW disks if I want to be able to access the data in a long while - use CD-R or DVD-R ones for the best reliability. Some disks are rated achival, or for the data to be retained longer otherwise (e.g. Fuji Photo CD-Rs).