In the early PC days, letter-writing was still common, and that was at first the main channel of communication to report issues. When CompuServe took off that became the preferred forum, at least in the US. Phone support was also always an option!
Here's a typical contact section from a manual, in this case the 1987 Turbo C manual:
How to Contact Borland
The best way to contact Borland is to log on to Borland's Forum on CompuServe: Type
GO BOR
from the main CompuServe menu and select “Enter Language Products Forum” from the Borland main menu. Leave your questions or comments there for the support staff to process.
If you prefer, write a letter detailing your comments and send it to:
[...]
As a last resort, if you cannot write to us for some reason, you can telephone our Technical Support department. Please have the following information handy before you call:
- product name and version number
- computer make and model number
- operating system and version number
(the phone numbers were provided in a separate document in the box).
As mentioned in the answers to How were analytics gathered on software built for retrocomputing platforms?, when that stopped scaling, users wrote to magazines instead, in the hope that their problem would grab the magazine writers' attention and they would either solve it themselves, or use their contacts in the relevant company. Actually that's not quite accurate, users started writing to magazines with programming problems as soon as magazines existed!