Just as "More is more" is an obvious concept to understand, so is "Less is less". Not to be confused with "Less is more", when doing less can sometime be a positive thing if you are edging towards doing more than you can handle or adapt to. Doing less in that scenario can be a somewhat of a challenge for some people who are highly motivated, even if there are strong benefits of doing so.
Less is less can be a situation you find yourself in not through choice; injury, sickness, work, motivation and other forces can come into play that simply require you to do less than you want to do. In the short-term your fitness might not suffer, and maybe improve through the forced rest and recovery. But as the length of time stretches on where you are doing less, inevitably your fitness will deline. As simple as that.
The way you respond to this less is less situation is fully dependent on why you are doing less in the first place. If you are sick or injured, then get yourself better. If work is busy, home life is chaotic, or similar, only you can understand what you do to help the situation. Likewise if your motivation has gone...seek help on a response from those you know and trust. Address the reasons why you're doing less as the top priority, especially if they are causing you stress.
Returning from a less is less situation, and re-building your fitness, is similar to getting fit into the first place. Be patient. Your fitness will return. It might take time, and you might need a different approach, but stick with it. As with the other situations, less is less can merely be a period of time you are going through. Keep on working forwards and doing the best you can.