If you're sick -> take care of that first. Don't work.
If you're just having an off day, trouble focusing, not feeling in the mood, that's different. I can suggest a few things that's worked for me.
1) Focus on just doing one thing.
Pick your most important project, then pick one specific piece of that, and shut down everything else until you at least complete that.
2) Set a timer.
If you're problem is getting started on a new or challenging project, do something like the pomodoro technique where you just promise yourself you'll work on it for X minutes and then stop. This removes some of the barrier to entry and can help you get started.
3) Remove distractions.
Similar to the first point, but is Slack always on? Are you getting notifications from your phone? Checking e-mail? This is a problem with our current life, we have too much competing for our attention. A lot of what we need to do to be successful is blocking out the noise so we can focus.
Hope this helps!