Ctrl U thanks Crist
In my opinion everybody spamming Ctrl Z for deleting the previous step.
z is next to u
:)

command + ; fixed my issue
View > Guides > Show Guides
Seems like being able to "see" the Guides should be the default when you first install.

Illustrator 24.3, Mac OS Catalina 10.15.7

Smart guides have stopped working.
I.e Smart Guides do not appear when drawing using the Line segment tool, or rectangle tool for example.
Smart Guides do not appear when aligning object with other objects.

I'm running the same release (24.3) on a PC, Win10 64-bit, and do not have the same problem on that machine.

Smart Guides are enabled - were have they gone?!

I have the same issue, started all of a sudden. This morning it was working, and now it just is not working anymore, I did not change anything in the Settings... Very strange and annoying.

Having this issue. Need this feature! I have it selected/on - no guides appear! Is Adobe working on this?

Having this issue after upgrading to 24.1.1 Win 10

I'm having the same problem. Adobe needs to fix this ASAP.

In Preference/Smart Guides, make sure you have Alignment guides selected and a bright color as well!

control + U only brings up the right click menu. I have not been able to get smart guides to show up or work and I've tried everything suggested in the comments below. Using Illustrator 2020 24.0.3

Control U - of course- which goes to show when soo many users have issues with your design (hello adobe), fix it!
You have had a number of hot key patterns over the years that work like **** and waste users dev time. Users constantly hit this miss-mapped hotkeys and have no idea whats happened to a function. Adobe makes a lot of cash on the subscription model now - use the money to fix the existing infrastructure. Stop making a new version/update very 6 months. Fix the old issues! This has ****** product behavior has become acceptable much like car recalls or software updates. Just put your **** product out there now (to hit deadlines) and we will fix it later...

Having the same issue now. Tried command c and command u, none of them worked.

Hit control + U

As others have mentioned below, if you have 'Smart Guides' turned on and any of the 'Snap to...' features turned on at the same time your Smart Guides may not work. I turned off 'Snap to point' after experiencing the same problem and suddenly it works again. I'm currently using CC 2018 as I've seen CC 2019 is still having bug problems!!

Hit control + U .
Solved for me

Adding Guides are still a very hit and miss exercise! Sometimes they are placed, mostly they are not, especially at high Zoom nothing happens when dragging a Guide from the Rulers! Also, at 100% everything looks fine but when magnified the shapes are not snapped together exactly. When they are accurately aligned, and then rotated, or moved they become misaligned. Such a cumbersome and inaccurate Drawing programme.

i have also had this now issue since the last update!

cmd + k
Smart guides -> Snapping Tolerance 4pt

For me worked

I had the same issue and Guide menu option (view>guides>) was disabled too, then I realised that I have Trim view was on. Make sure it is off and you will see the guide again, this is my case. Also make sure guides are not hidden.

Mine isn't working either + I've tried so many ways to fix it. All these bugs + problems are SLOW DOWN MY WORK!! Everything was fine with my illustrator until I did the update!

I had the same issue and just turned off and then on again the smart guides option and it worked again. Version 2018.