Most likely some addon has disabled the Blizzard addons that provide the default UI's compacty party and raid frames. You can't turn them back on via the addon list, but you can turn them on by calling an API function in addon code (or in a macro). However, before you do that, you need to figure out which addon is disabling them, or it will just do it again. It's most likely an addon that provides its own party and/or raid frames, or an addon whose specific purpose is to hide the default UI's party and/or raid frames. If you have no idea which of your addons might include such a feature, here's how you can figure it out:
Option #1:
If you're not afraid of seeing Lua code, you can take a shortcut. If you don't already have a program that can search for text inside files,
grepWin is a good one. Search for "DisableAddOn" in all *.lua files in your AddOns folder (and subfolders). You'll get about a billion matches on all the copies of AceAddon-3.0 in various addons. Ignore those. Sorting the results by file name will help you ignore them.
If you see any lines that appear to be disabling a Blizzard addon -- eg.
DisableAddOn("Blizzard_CompactRaidFrames") -- then you've found the problem. Look through that addon's options to see if there's a way to stop it from hiding the Blizzard frames. If not, then your options are to stop using that addon, to start using an additional addon to replace the Blizzard frames, to ask its author to add an option and hope they will, or remove the offending code from the addon. If you need help with that, post the name of the addon here and we can help you.
Option #2:
If you're scared of seeing code, or don't have anything that can search in files and don't want to install anything, here's the long way:
1. Exit WoW and delete all of the the "AddOns.txt" files in each character's individual folder in the WTF folder. This will reset the enabled status of all addons, including the Blizzard ones.
2. Launch WoW again, but disable all of your addons at the character screen
before logging in. Verify that your party/raid frames are working.
3. Now look over your addons list. If you use any custom unit frame addons (raid frames, party frames, player frame, target frame, etc.) enable
one of them. Log in. Do you still have party frames?
4a. If not, then you've found the problem. Look through your unit frame addon for an option to
not hide the Blizzard frames. If there isn't an option, then your choices are to stop using the addon, or complain to its author and hope they add an option for you, or install another addon to replace the missing Blizzard frames.*
4b. If you do still have party frames, keep enabling addons until they disappear. Once you find the problem, see Option #1 for your options.