Nothing changed but team performs very bad all of a sudden

[eXtEnDeD]

Set higher win bonus, fill up the morale, substitute some players which don't perform good recently and try to focus your play where are your well-performing players. If you cannot score a lot of goals, set up some trainings for your STs, etc
There are plenty of things you can get on winning streak, good luck

[TheWhiteKirby]

It would be better if you told us your tactics. What's your pass focus? Pass style? Formation? Team Mentality? I have had these spells myself but very briefly. What I do to turn games around that aren't going my way is to watch the live ratings of my players and my opponents' players. See where I am doing bad and where they are doing well. I also study their formation for weak points and their players' ratings for poor players. I focus my pass formation around the area where their players are performing poorly and that usually helps turn games or results around. Don't be afraid to be unorthodox as well, if you are losing and its the 55' minute sub a striker in for a winger or an AMC, they may be out of formation (mine aren't as I have all multi-pos players in my squad just personal choice), but they may also get that last gasp goal you need to get a point or all 3 points out of a fixture.