Can you explain the status of a mursal hadith if it has sahih chain and the narrators are thiqah can it be used as evidence to prove something in Islam or is it regarded as a weak source? I believe the mursal hadith is narrated from tabi'een
The short answer is it depends, but generally speaking not authentic.
The definition of a hadith mursal is that the tābi'i (Arabic: التابعى), which is the generation after the companions of the Prophet ﷺ starts a narration with: "The Prophet ﷺ said ... ." It is a type of hadith maqtū' (severed chain of narration), at the companion's level.
المرسل علم على ما سقط ذكر الصحابي من إسناده فيقول التابعي قال رسول الله صلى الله عليه وسلم
Hadith mursal is an indication that the companion's layer has been dropped from the narration chain so that a tābi'i says "The Messenger of Allah ﷺ said ..."
The general rule is that if a hadith does not have a continuous narration chain from the narrator to the Prophet prayers be upon him, then this is a weak hadith (or lower) because of the issue with the narration chain. In the case of hadith mursal, there are some exceptions:
Tābi'i kabir (highest level): Sa'īd ibn al-Mussayib, Masrooq, Al-Sunabiji, Qays ibn Hāzim, etc. These are generally accepted as authentic (especially by Maliki and Hanafi schools) if there are no other reasons to reject the hadith in terms of its narration chain.
Tābi'i mutawassit (middle level): Mujahid, Al-Shua'bi, etc. These some scholars accept and some reject.
Tābi'i sagheer (lowest level): Al-Zohry, Al-Hassan, Qatada, etc. These are mostly considered weak or worse.
It is important to note that the classification of highest, middle and lowest levels of tābi'is has nothing to do with their character or their dependability in hadith narration. It is related to the number of their teachers that were from the companion's level. For example, Sa'īd ibn al-Mussayib studied from over a hundred companions, whereas Al-Zohry studied from less than ten companions. But both in terms of the science of men are of the highest authenticity levels as narrators.
Note that there are two additional types of mursal related to the companions, and they are also generally accepted as authentic. There are lots of hadiths where there are two or more companions in the narration chain, especially noted with Abdullah ibn 'Umar and 'Umar ibn Al Khattab, e.g., the hadith of Islam, Īmān, and Ihsān, or the hadith about deeds being by intentions. This case is also when the companion starts the narration by: "The Prophet prayers be upon him said ...", instead of, for instance, "I heard the Prophet prayers be upon him say ...". This applies to the companions that narrated the hadiths of the Prophet through another companion or those who saw the Prophet but did not hear from him directly (e.g., Abu Umama ibn Sahl) and narrated their hadith's through another companion. I am assuming, though, that you are not asking about these types.
There are several types of hadith mursal that you may want to study from a reference book on hadith (e.g., Muqaddimat ibn Al Salah, Al-Muqitha, Alfeyyat Al Suyuuti, Alfeyyat Al Iraqi, etc.). All these references consider these other types as weak or worse. The irsal in these cases may be explicit, or hidden (irsal khafi) related to continuity, places, or times.
Al-Mūqidha in explaining the science of hadith (Shams ad-Din al-Dhahabi), Arabic version.